Recently, the number of pixels of the imaging element for a digital camera increases, and according to this, minimization of the pixel of the imaging element develops. When the pixel of the imaging element is minimized, a saturation charge amount of each pixel becomes smaller, and as a result, deterioration in an S/N ratio becomes significant and an image quality is deteriorated.
Although it is tried to remove noise by signal processing against the deterioration in the S/N ratio, the noise and a signal cannot be completely separated by such signal processing and this often causes the deterioration in the image quality. Therefore, in order to maintain excellent image quality even when the saturation charge amount decreases due to the minimization of the pixel, it is desirable to allow light to be incident on the pixel as much as possible. For this, a method of enlarging a bandwidth of a wavelength, which is incident on the pixel of the imaging element, is considered.
In a color imaging device, which uses the imaging element such as a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS), in general, an infrared cut filter for cutting an infrared component in incident light is arranged on the front of the imaging element in order to improve color reproducibility.
However, when the infrared cut filter is used, an amount of light incident on the pixel decreases. Therefore, when it is wanted to obtain a sufficient amount of light, the light out of an infrared region is allowed to enter without using the infrared cut filter, however, in this case, the infrared component is included in the incident light and the color reproducibility is significantly deteriorated.
A reason of the deterioration in the color reproducibility when the infrared component is included in the incident light will be described with reference to the drawings. In general, one color filter for one pixel is attached to the imaging element. FIG. 7 illustrates a general filter arrangement. As illustrated in FIG. 7, the filters attached to the imaging element have a so-called Bayer arrangement in which three elementary colors of red (R), green (G), and blue (B) are used. FIG. 8 illustrates spectral characteristics in elementary color filters. FIG. 9 illustrates the spectral characteristics in the elementary color filters when using the infrared cut filter. Relative to the incident light as illustrated in FIG. 8, a part with a longer frequency than an infrared cut frequency is cut as illustrated in FIG. 9.
In a general image signal processing of a camera, white balance adjustment is performed such that values of R, G, and B are the same when a white object is photographed. If values of RGB before the white balance adjustment are set to Rorg, Gorg, and Borg, white balance adjustment gains are set to Rgain, Ggain, and Bgain, and the values of RGB after the white balance adjustment are set to Rwh, Gwh, and Bwh, following equations are obtained.Rwh=Rorg×RgainGwh=Gorg×GgainBwh=Borg×Bgain
With ideal white balance adjustment gains, Rwh=Gwh=Bwh is satisfied when the white object is photographed. FIGS. 10(a) and (b) illustrate change in signal amounts of RGB at the time of ideal white balance.
When there is no infrared cut filter, depending on a light source and a subject, a signal of the infrared component is accumulated as illustrated in FIG. 11(a) also for the white object. The signal of the infrared component does not have correct RGB balance, so that by multiplying the white balance adjustment gains, Rwh=Gwh=Bwh is not satisfied as illustrated in FIG. 11(b) and coloring occurs. For a chromatic subject other than white, the balance of R, G, and B is similarly disrupted by an effect of the infrared component and the white balance adjustment. For these reasons, the color reproducibility of the camera is deteriorated when the infrared component is included in the incident light.
Some methods are suggested for overcoming the above-described problem. An imaging device disclosed in the patent document 1 has a configuration illustrated in FIG. 12. The imaging device realizes improvement in color reproducibility for a bright subject and improvement in sensitivity for a dark subject by putting on the infrared cut filter in the day time with sufficient illumination intensity and removing the infrared cut filter from the front of the imaging element and selecting a black-white signal to output based on a result of the signal processing of a video signal obtained from the imaging element in the night time without the sufficient illumination intensity.
Also, an imaging device disclosed in the patent document 2 has the RGB pixels and an IR pixel on which visible light and infrared light are incident, as illustrated in FIG. 13, and generates an image using the RGB pixels at the time of high illumination intensity and generates the image using the IR pixel at the time of low illumination intensity, thereby realizing the improvement in color reproducibility for the bright subject and the improvement in sensitivity for the dark subject.